Ministry of offence: Armando Iannucci takes on the White House

The setting: a room, deep in BBC TV Centre with "Armando Iannucci" on the door. Here, The Thick of It was written – our twisted British answer to The West Wing and the defining political satire of the past decade. Here, they made up mad Policy Enforcer Malcolm Tucker (Peter Capaldi), stitching him together from bits of Alastair Campbell, Basil Fawlty and Hannibal Lecter. Here, they wrote his legendary response to a knock at the door: "Come the fuck in or fuck the fuck off."

Here, too, they have just made The Thick of It's big-screen "cousin", In the Loop. It has all the "insider", scabrous qualities of the TV version, plus some cinematic high-voltage courtesy of new cast member James Gandolfini, who plays a US general, a dove in full military regalia. How did the writers gather the information, infiltrate the UN, and think up yet more sentences in which every word is profane?

This is the place to find out. Props inside the room include: Jammie Dodgers (one packet), kettle, whiteboards covered with notes on next year's series of The Thick of It, and a huge diagrammatic wall chart on dynamic team-building (ironic). Centre-stage, controlling the computer and the general atmosphere, is presiding genius Armando Iannucci (AI); also present are the writers Simon Blackwell (SB), bearded, a friendly comedy pro who has contributed to shows from Have I Got News For You to Moving Wallpaper; and Tony Roche (TR), younger, more diffident, very tall. All three are wearing suits.

AI We're usually here in our pyjamas...

SB Romper suits.

AI But this is dress-up Thursday. For The Independent on Sunday. Isn't the idea to recreate one of those tense White House photo shoots, like the archive ones of LBJ?

[Squeals of laughter audible from the office next door]

AI That's just Strictly Come Dancing. It's always like this.

TR On the set of In the Loop you could sometimes hear the actors trading lines and warming up. You'd be sitting in the writers' room, and you could hear Peter Capaldi in the Portakabin next door, shouting "puppy fucking machine" again and again. As his warm-up exercise.

SB Or singing Sinatra.

AI We usually had one writer on set at a time, in a rota.

SB Sometimes we paired up. But it's not like we sit there trading zingers. Mainly just writing, grunting it out.

AI I would ask whichever writer was on duty that day to contribute 10 things that each character might say. So Gina McKee [playing press secretary Judy Molloy] would get 10 phrases that morning that she can use whenever she likes.

SB One of them is in the film, at the beginning. "You're co-ordinating the project? Co-ordinate it better."

AI None of the rest of the cast is expecting them.

SB It keeps them fresh.

[Knock at the door. Enter co-writer Jesse Armstrong, also co-writer of all things Mitchell and Webb, including 'Peep Show'. He is carrying a furled periodical under his arm – 'The New York Review of Books' – and is wearing a suit]

JA Ah ha ha. Look at us!

AI It's Gilbert and George and Gilbert.

SB And Gilbert's friend.

AI So, first of all, I divided the film into four acts and each of us took one. Then I put some notes in and we went round and read everyone's. I always say, write it very quickly, because it's not about finessing. On TV, every episode is only 20 minutes long whereas here the script was...

SB Fucking massive.

AI Most scripts are 90 pages but ours was hitting 300. Then it gets roughed up with improvisation. The first edit of the film was four-and-a-half hours' long. Then it all gets cut.

JA If you think about it, there's roughly a 10 per cent chance of anything you write making it into the film.

AI That's why I say, just write as quickly as possible.

JA Yes, it's nice that Armando takes the pressure off, says "chuck it in". But, as a writer, you can't chuck any old thing at Armando Iannucci. My dog's got no nose – that's not OK.

HE Do you mourn the bits that don't make it in? '

SB If you did, you'd be in permanent mourning. You've got to let it go. Otherwise you'd go mad.

JA Ian and I were reminiscing about bits not in the film.

AI Like old veterans. Do you remember scene 59?

HE Is that Ian Martin? The Thick of It's swearing consultant?

JA He's more than that. As a title, it's unfair to him, because he does more than that, and it's unfair to us, because we do a lot of good swearing.

AI He's writing his own episode of The Thick of It. He doesn't live in London. He's in the building trade.

HE So you can't really pinpoint who does what?

SB It's more like there's a house style. A hive mind.

HE You all bring in different elements – cuttings, diaries?

AI Diaries are very useful. When people left office, it used to be that they had to wait 30 years before publishing. Now it's all freely available. There was a passage in Clare Short's diaries about the Iraq war that I thought was just mad, where she said, "In the end, I concluded it would be braver not to resign." I wanted to work out how someone probably pretty decent can end up just tying themselves in those moral knots.

SB We all read the papers.

TR We watched No Plan, No Peace [John Ware's Iraq war BBC documentary] which was excellent for seeing the neocon tone.

AI And Sean [Gray, a researcher on the film] and I went to Washington. We got into the State Department using this BBC pass. I mean, this pass barely gets you into the BBC... We'd been told to up to reception and say: "BBC, I'm here for the 12.30." And we got in. There we were, wandering around the State Department. We took our phones out and started taking photographs so the set designers would have something useful. A part of me thought, this is fun. A part of me thought, this is international espionage, so it might carry a 20-year penalty. A big guy came up to us and said, [menacing voice] "'Scuse me?" And we said "We're here for the 12.30."

SB We worked through all the stuff they brought back.

TR Used it as a jumping-off point.

AI "Where did you get your BS degree?" I overheard that.

JA It was brilliant – mundane details you'd never have thought of, like in one scene they go for a meeting and there's a table- tennis net across the table. Perfect.

TR The hand grenade as a paperweight is a real-life thing.

AI Yes, Senator John Bolton has a hand grenade as a paperweight, which is mad. And the place is overrun by 22-year-olds.

HE Some of your lines are spookily prophetic. For instance, the one about people being banned from shops if they don't speak English – it's uncomfortably close to the recent controversy about people having to speak English in post offices.

TR I always find that worrying. You're trying to think of something slightly sillier than reality – and then it happens...

AI I once wrote something about a minister walking to the House of Commons, with his dispatch boxes following in a car after him, because they're Crown property. I used it for auditions and chucked it away after because it was so silly. Then came Cameron on his bike, with his car behind him...

HE What else did you sneak out of Washington?

AI We interviewed lots of staffers – but not surreptitiously. Many of them knew The Thick of It. We said, we're not out to expose anything, it's a fiction, an entertainment. What I want to know is the dull stuff; I love it when they tell me things like how you can spot the Democrats from the Republicans: Democrats come in late and go home late, and Republicans come in early and leave early.

TR Bush took that to the nth degree.

AI He was at his desk at seven in the morning.

TR And asleep at eight in the morning.

AI Lots of little details come together. Like Post-It notes. All the emails and phone calls at the White House are on the record, but Post-It notes aren't, so all the important stuff goes on them. Like: "Kill Castro".

SB Or "Buy Milk".

JA And... that's how they ended up killing Harvey Milk.

SB Jesse done a joke!

[TR finds the button on his phone that makes a cymbal crash]

HE Could you ever have a woman writer on your team?

AI What a strange question.

TR Maybe for a cuddle.

AI I liked Bob Woodward's descriptions of these big meetings in the build-up to war in Iraq. He said no one was brave enough to ask, "What happens once we've invaded?" And if they did, everyone would just look at the floor. You know how at school when you don't know the answer, you pretend if you don't look at the teacher, they won't be able to see you...

SB We've always wanted to show the human being behind the politician. If you had all those pressures piling in on you, maybe you would take that decision too.

HE So it's not intended as a bitter indictment of politicians?

AI The faults are not just with the politicians but with the pressure put on them by the media and the appetite of the people who buy the newspapers, so we're all implicated, really.

TR You need to feel warmth and empathy for the characters otherwise you wouldn't want to spend any time with them.

HE Jesse, you used to work for a Labour MP – are you the only one here who's worked in politics?

TR Yes, although Simon's hoping to get into politics.

JA I worked at a low level 10 years ago as a researcher for a nice Labour MP. The thing I remember most is how you become your job. You start wearing the same suits as everyone else and walking two paces behind the guy you're working for... It's a very febrile atmosphere. Often, those at a young level haven't been massively successful socially and then they find a home in a particular party and, well, they live it, don't they?

AI Sometimes the script seems more satirical and then the performances humanise them. We do the casting quite early.

TR It's always easier to write after it's been cast and you know who you're writing for. You do all the voices, don't you Simon?

SB Yeah, I speak a lot under my breath while I'm writing.

TR Chris Langham had this amazing, unique vocal rhythm.

HE Is there any chance of Langham returning? [His character Hugh Abbot is on a secondment to Australia in the series]

[Quiet falls]

AI I don't think so. We keep in touch, but he knows that the BBC won't even repeat the episodes he was in. I think it's stupid, because I'd really like to see him back in something.

HE Did you ever disagree dramatically on anything?

JA Armando is the boss. I think if at any point one of us had got up and said, "I think that war in Iraq was the most magnificent idea ever" – that might have been a problem.

AI I can tell, reading those scripts that come in, that one of you is right-wing.

[Howls of laughter]

TR Is that the script that comes in earliest?

AI I look down and see, yes, Rightie has filed.

SB There were no disagreements as such – it's more just working out what works and what doesn't.

AI And making it easily comprehensible without recourse to explanatory voiceover. You always know a film has gone wrong when you need those. We toyed with having a Star Wars-esque kinda rolling-text explanation: "Countdown to war..."

SB But it would have lasted 40 minutes.

HE Didn't you film in Downing Street? How was that?

AI Weird. Really weird. Everyone wanted photos with Peter Capaldi. Even Alistair Darling's son.

HE But you didn't manage to film in the UN building?

AI No, the UN has cottoned on to the whole BBC pass thing. But I did go there for research. There's signs saying "No Smoking" everywhere, but everyone is smoking. The French started it, apparently. You couldn't throw France out of the UN for smoking, could you?

[Enter publicist Rupert; he tells Iannucci about a screening of 'In the Loop', where he'll need to introduce the film]

AI With a speech?

SB Or some impressions maybe.

AI Perhaps I'll do something from my student days: Pope John Paul II singing Julie Andrews. [AI starts to sing, in dirge-like voice] Sooper-Caali-Fraagilistic-Expialidocious...

[Exeunt, pursued by publicist]

'In the Loop' (15) is in cinemas nationwide from Friday

The reshuffle

"It was always my intention that In the Loop should be self-contained," says Iannucci, "so you could enjoy it even if you didn't know The Thick of It."

The Americans are coming... the Washington delegation:

IN

James Gandolfini: The Sopranos star turned out to be a fan of The Thick of It, as well as deeply informed about the war in Iraq. (He interviewed veterans for the HBO documentary "Alive Day Memories".) Top quote: (menacingly, pointing at Malcolm Tucker's forehead) "That's where I'd put the bullet... You look like a squirter."

Mimi Kennedy: The actor (Pump Up the Volume, Dharma & Greg) and activist plays Karen Clark, the ballsy US Assistant Secretary for Diplomacy. Top quote: "I want a head on a plate to go."

Anna Chlumsky: Starred with Macaulay Culkin in My Girl as a child. Plays intern Liz Weld. "She was great at improv," say Iannucci. "You could just leave the camera running and walk away."

New British cast members:

IN

Tom Hollander: plays a junior minister who tries to win diplomatic prestige on a mission to Washington, forgetting his constituents' more pedestrian needs in the process. Top quote: "I really hope there isn't a war – it's bad enough having to cope with the fucking Olympics."

Gina McKee: (Our Friends in the North, The Forsyte Saga) is the put-upon, world-weary director of communications Judy Molloy. "She's so brilliant, I don't know why she doesn't do more comedy," says Iannucci.

OUT

Chris Langham: won a Bafta for his performance as minister Hugh Abbot in The Thick of It, but has not worked for the BBC since being found guilty of downloading child pornography in 2007.

Justin Edwards: The actor who played ministerial dunce Ben Swain does not appear. "A shame as he's very funny," says Iannucci.

"Ollie Reeder": The hapless wonk of the TV series is now called "Toby Wright", but is still played by Chris Addison.